core: improve named icon detection

[charlesrocket]

This fixes weak named icon detection. The provider now can look for light or dark icons depending on the system settings (KDE/GTK).

Valid paths (that were undetectable before):

/usr/local/share/icons/breeze/status/24/transmission-tray-icon.svg
/usr/local/share/icons/breeze-dark/status/22/transmission-tray-icon.svg

Now the provider will pull the icon from breeze-dark if dark icons are preferred.

Tested on FreeBSD via transmission-gtk/transmission-qt.

[outfoxxed]

This is generally supposed to be handled by qt, and the qpa selecting light or dark icons.
We do eventually want to rewrite this, but a more complicated rewrite is required here. Notably it needs to be async and not hardcode anything but fallback paths.

[charlesrocket]

Pulling the GTK preference would probably require a hardcoded path to config, not sure if I can reach it via Qt API.

[outfoxxed]

This shouldnt require checking the gtk theme. If the user configured Qt properly, then the information should be available from the QPA.

[charlesrocket]

You mean GNOME settings to KDE settings configuration?

[outfoxxed]

No. Most features of Quickshell don't even work under GNOME since they don't implement the protocols necessary for it, and I'm not aware of anyone using it under KDE, as plasmashell has much better insight into the DE.

I'm talking about standalone QPA themes such as qtengine and qt(5/6)ct, or KDE's QPA, which are supposed to back Qt's icon theme loading, including color selection. For example my icon theme is "breeze-dark" set by qtengine.

[charlesrocket]

Oh I see, I'll check it out.